1. Technical Field of the Invention
This invention relates to a method for recording holographic images of human subjects safely and in a manner whereby dark eye adaptation of the subject is minimized and to obtain the recording of more lifelike human images.
2. Description of the Prior Art
The recording of images by a holographic process necessarily requires a laser source to generate the object beam and the reference beam. The energy generated by a laser must be of sufficient intensity to record a holographic image on a holographic film plane.
Particularly when the object being recorded is a human subject, the geometries of the object and reference beam paths should be such that any risk that the subject be exposed to laser light be maintained at a minimum. In fact, present laser safety regulations have set the exposure levels based upon what can be safely tolerated by the human eye. (ACGIH (1972), American Conference of Governmental Industrial Hygienists; ANSI (1973), American National Standard for the Safe Use of Lasers).
In holographic applications, the object beam is normally expanded by the use of lenses and then diffused by the use of ground glass screens or the like. If the object beam is not properly expanded or diffused, it may pose a hazard to a human subject. Also, the reference beam, which must be undiffused because it necessarily must have a higher energy level to facilitate recording of a holographic image, also poses a potential threat to human subjects. Various geometries such as an overhead reference beam give partial safety, but do not absolutely safeguard against accidental or deliberate viewing of the reference beam by a human subject.
Another consideration when recording holographic images of human subjects is to obtain the most lifelike reconstruction and clarity of the image. However, at the instant the holographic image is recorded, the room should be generally free of light sources other than the laser light which generates the object beam and the reference beam. When using a pulse laser to record an image, it is generally desirable for the subject to sit in a dark room up until the moment that his image is recorded. However, as is well known, the pupil of the eye varies in diameter in response to the illumination level. Accordingly, the prior art attempts to record holographic images of human subjects under dark room conditions have resulted in reconstructed images whereby the subject's pupils have been unusually large in size which is unlifelike.
Accordingly, it would be desirable to provide an apparatus and method for recording holograohic images in human subjects which is both safe and which results in natural and lifelike images being recorded.
This invention is directed to a method for recording lifelike images in holographic recording materials and more particularly to a method whereby dark eye adaptation of a human subject is minimized. Experiments have shown that the pupil of the human subject varies within the extremes of 2 and 10 mm. When the human subject is subjected to a change of illumination from bright to darkness, the pupil diameter will change from less than 3 mm to over 7 mm over a period of about 100 seconds. Tests have shown that the pupil will commence to respond to darkness after only about 0.5 seconds. (See generally, Light Color & Vision, Vyes & Grand, Chapman & Hall, pp. 96-98). Therefore, in order to record a lifelike holographic image of a human subject, when the subject is initially illuminated with natural room light, it is necessary that the recording event take place substantially within a 0.5 second interval after white light illumination of the subject is terminated. It is desirable to maintain the subject in natural room light prior to the recording event so that a holographic operator can choose the subject's expression under the most natural of conditions, i.e., white light. Accordingly, the present invention provides a method of recording a holographic image of a human subject which provides for illuminating the subject in white light up to the time of the recording event, but which also minimizes the possibility of dark eye adaptation of the human pupil.
When the pupil of the subject's eye is dark eye adapted (having a pupil diameter of 7 mm), the maximum permissible safety levels for a Q-switched ruby laser, based on the ACGJH 1972 and ANSI 1973 formula (Handbook of Optical Holography, Caulfield, Academic Press (1979), p. 616), is I.sub.Rmax 0.07 J/cm.sup.2 (which resolves to I.sub.Dmax =0.07 J/cm.sup.2), for the energy density incident upon the diffusing screen. However, for a daylight adapted eye having a diameter of 3 mm, this value can be increased by a factor of 5 to 0.35 J/cm.sup.2, since the I.sub.Dmax =0.07 J/cm.sup.2 calculation is based upon a dark adapted pupil with a diameter of 7 mm. This increased energy density on the illumination screen allows for more spot-like illumination effects as opposed to the flat diffused lighting required by the 0.07 J/cm.sup.2 level, or on the other hand, if conventional lighting is still used, the safety factor is increased by a factor of 5. Therefore, not only do subjects with daylight adapted eyes appear more lifelike, but in addition, the energy density can be higher and still be within safety levels.
In accordance with the method of the present invention, the holographic recording medium has a shutter and uses a pulse laser which generates an object beam and reference beam. The method of recording comprises the steps of illuminating the subject with a white light source for at least a predetermined time period, extinguishing the white light source, opening the shutter mechanism of the holographic recording medium, activating a pulse laser having an object beam directed towards the subject with the pulse laser positioned so that the object beam light reflected off the subject to be directed towards the recording medium, said laser also providing a reference beam directed towards the recording medium to thereby record holographic image, and closing the shutter mechanism. It is preferred that the steps of extinguishing, opening, and activating occur within the time period of about 0.5 seconds, so that dark eye adaptation of the human subject will be minimized.
By practicing the method of the present invention, the holographic operator will know exactly the expression of the subject's face at the moment of the recording event, i.e., exposure of the holographic recording medium, and the continuous illumination of the subject will prevent dilation of the subject's eye which would otherwise occur due to dark eye adaptation. Also, by providing for the above steps to occur within a time period of substantially 0.5 seconds or less, not only is eye dilation minimized, but subject anxiety is lessened, which will contribute to a more natural and lifelike image being recorded. Also, a red filter can be inserted over the white light source so that the subject is illuminated in the same color light within which his image will be recorded. In this way, the holographic operator will be able to see the subject under the same color light conditions which exist during the actual recording event.
Numerous other advantages and features of the present invention will become readily apparent when the following detailed description of the invention and embodiments thereof, from the claims and from the accompanying drawings.